The ideal operational amplifier (A-level only) Flashcards – AQA A-level Physics (7408)

What does an operational amplifier amplify?

The voltage difference between its non-inverting and inverting inputs.

What are the three signal connections of an operational amplifier?

A non-inverting input, an inverting input and an output.

What is the role of an operational amplifier’s power-supply connections?

They provide the energy needed for operation.

Relative to what are an operational amplifier’s signal voltages measured?

A common reference.

What is the input resistance of an ideal operational amplifier, and what does it imply for input current?

It is infinite, so no current enters either input.

What is the open-loop gain of an ideal operational amplifier?

It is infinite: any non-zero input difference would demand an arbitrarily large output in an unconstrained linear model.

What equation relates the output to the inputs of a real operational amplifier in its linear open-loop region?

$V_{\mathrm{out}} = A_{\mathrm{OL}}(V_+ - V_-)$, where $A_{\mathrm{OL}}$ is the finite open-loop voltage gain.

What limits an operational amplifier’s output in practice?

Its power supplies.

What does an open-loop operational amplifier used as a comparator indicate?

Which of its two input voltages is greater.

How does a comparator’s output depend on the relative sizes of $V_+$ and $V_-$?

• If $V_+ > V_-$, the output tends toward positive saturation.
• If $V_+ < V_-$, it tends toward negative saturation.

Why does an open-loop comparator’s output tend toward saturation?

Its open-loop gain is large.

What limits an operational-amplifier comparator’s saturation levels?

The power supplies.

What happens when a comparator’s signal voltage crosses its reference voltage?

The output changes state. Which input receives the signal determines the output polarity above the reference.